1. Technical Field
This invention generally relates to semiconductor devices, and more specifically relates to fabricating resistors in semiconductor devices.
2. Background Art
The use of integrated circuits has spread to all types of products for many types of uses. These integrated circuits contain a wide variety of components, such as field-effect transistors, bipolar transistors, etc. Some of the necessary but also the more difficult and space intensive components to fabricate are resistive elements or resistors.
The types of resistors which can be fabricated in integrated circuit (I.C.) processing can be classified into three categories: (1) diffused resistors; (2) field effect resistors, and (3) thin-film resistors.
Diffused resistors are created by doping the semiconductor device with n or p type materials and annealing the semiconductor device. The resistance of the diffused resistor depends upon the length, width, depth of the diffusion and resistivity of the diffused material. The resistivity of the diffused material is dependent upon the dopant and junction profile of the dopant species after a high temperature anneal.
Field-effect resistors are transistors used in depletion-mode and modulated by the implantation that controls the threshold voltage of the transistors.
The diffused resistor and field-effect resistor each have the advantage of being easily integrated into the I. C. fabrication process. However, these types of resistors also have numerous disadvantages. Specifically, these types of resistors are usually limited in resistance and tolerance because the parameters controlling these resistances are the same parameters that must optimized for device performance. Thus, only low resistances are available with practical I.C. diffusion structures or field effect transistors. Additionally, the diffused resistors have the drawback of requiring excessive space on the I. C., a serious disadvantage as the size of I.C.s continues to shrink.
Conversely, the third category of resistors, thin film resistors, are typically resistive metal films which are deposited upon dielectric materials such silicon dioxide (SiO.sub.2). These thin film resistors have the advantage of sparing critical area upon the silicon (Si) substrate. Unfortunately, these resistors are typically produced only in Back-End-Of-Line (BEOL) processing and are thus limited in application.
There are several metallic films that can be used in thin film resistors with relatively high impedance (1000-10,000 ohms/sq.). For example, thin film resistors can be created using metals such as cermets (Ni/Cr, Cr/SiO.sub.2) and rhenium. Unfortunately, these resistor materials are not commonly used in standard VLSI processing, and thus are difficult and costly to implement into the fabrication process. Additionally, the required metallurgical contact to these resistors as well as to Si devices may require different materials which may be incompatible in Si device processing.
Thus, the prior art suffers from the inability to make small and easy to manufacture resistors in I.C. processing. Therefore, what is needed is improved resistors capable of offering high resistance which can be easily integrated into conventional processing methods.